1. Field of the Invention
Monoliths of essentially uniform density containing thermoplastic particles essentially uniformly dispersed in a matrix resin system, are described. Composites of the monolith and core-holding structures are made by incorporating the monolith into the core-holding structure, optionally thermally treating the matrix resin of the monolith whereby the thermoplastic particles (if in situ-expandable) causes the thermally treated monoliths to faultlessly interface with the wall(s) of the core-holding structures. The invention relates to core supported composite structures, fasteners, adhesives, paneling, insulation, and other structures that employ the invention as a component.
2. Brief Description of Related Technology
Sandwich construction forms a major part of aerospace manufacturing; it is employed to some degree in almost every type of flight vehicle. Lightweight structural panels and panel systems for a wide variety of applications are industrially offered. They utilizing various laminating techniques to adhere thin, stiff “skins”, such as aluminum, steel, hardboard and fiberglass onto lightweight core materials, like honeycombs, urethane foams and balsa. The resulting panel is lightweight and strong.
Honeycomb sandwiches, i.e., honeycombs, are preferred structures in the fabrication of lightweight structures typically used in the aerospace and other commercial markets. The core material is usually “sandwiched” between skins of aluminum or other high strength composite material. A bonding adhesive is used to attach the “skin” material to the honeycomb core. The resultant honeycomb panel offers one of the highest strength to weight constructions available. For instance, the floor panels of most airliners use the lightweight/high strength construction of honeycomb. Aircraft engine nacelles, flaps, overhead bins and galleys all are constructed from honeycomb core.
A honeycomb may be called a multicellular structure, and it may be made of paper, plastic, fabric or metal, and other materials. The core of the sandwich is the honeycomb, a structure composed of row upon row of framed cells, or holes or wells, resembling the honey-storage facility of a beehive and characterized by a hexagonal or rectangular shape. To each side of the core are bonded extremely thin sheets of metal, creating the sandwich, which is far lighter yet has greater resistance to bending than a comparable thickness of metal plate. Aluminum is the most extensively used metal, in both the core and the facing sheets, but the technique is applicable to a large number of metallic and nonmetallic materials.
For example, M.C. Gill Corporation offers the following honeycomb products to Boeing's specifications:
SpecificationNumberQ1M2Product Code Number and DescriptionBMS 4-7CXGillfab 4030 - Aluminum facings/aluminumhoneycomb core sandwich panel.BMS 4-10XGillfab 5040Z and 5042 - AluminumTy 1, Gr 1facings/end grain balsa wood core sandwichpanel.BMS 4-10 Ty 2XGillfloor ® 5007A and 5007B - Fiberglasscloth facings/end grain balsa wood coresandwich panel for aircraft flooring.BMS 4-17XGillfloor 4417, Ty I thru Ty VI and Drawing69B15779 (Ty V) - Unidirectional fiberglassreinforced epoxy facings/Nomex honeycombcore sandwich panel.BMS 4-20XGillfab 4409, Ty II and Ty III (Ty I isobsolete) -- Unidirectional graphitereinforced epoxy facings/Nomexhoneycomb core sandwich panel.BMS 4-23XGillfab 5424, Ty I and Ty II - UnidirectionalS-glass facings/aluminum honeycomb coresandwich panel.BMS 7-326XGillfloor 5433C - Aluminum facings/uni-directional fiberglass reinforced epoxy coresandwich panel.BMS 8-2XGillfab 1076A - Fire resistant polyester glassCl 1 Gr Acloth cargo liner.BMS 8-2XGilliner ® 1366 - Extremely high impact,Cl 2 Gr Apuncture, and fire resistant polyester glasscloth cargo liner.BMS 8-2XGilliner 1366T - Same as 1366 but with a 1Cl 2 Gr Bmil white Tedlar ® overlay.BMS 8-2 Cl 3XGillfab 1076B - Fire resistant polyester glasscloth cargo liner.BMS 8-100 Gr AXGillfab 1108 - Epoxy/unidirectionaland B, Cl 1fiberglass cargo liner.BMS 8-13 Ty 1XGillfab 1137 - Nylon resin/nylon cloth fuelcell liner.BMS 8-223 Cl 2XGillfab 1367 - Phenolic/S-2 glass cloth cargoliner with low smoke emission.BMS 8-223 Cl 2XGillfab 1367A - Phenolic/fiberglass cargoliner with low smoke emission.BMS 8-223 Cl 2XGillfab 7146 - Phenolic/S-2 glass cargo linerreplacement kits.BMS 8-223 Cl 4XGillfab 1367B - Phenolic/fiberglass cargoliner with low smoke emission.BMS 8-124XGillcore ® HD - Nomex honeycomb.BMS 8-262XGilliner 1566 - KEVLAR ®/polyester cargoCl 1, Gr Bliner - very light weight.1Q = Qualified to specification2M = Meets the requirements of specification
To illustrate some of the physical characteristics of commercial honeycombs, such as Plascore Inc., Zeeland, Mich., PAMG-XR1 5052 Aluminum Honeycomb literature provides:
Typical Property ValuesPAMG-XR1 5052 Aluminum HoneycombPlate ShearStrengthStrengthModulusModulusHoneycomb DesignationBare Compression(PSI)(PSI)(KSI)(KSI)CellFoilDensityStrengthModulus“L”“W”“L”“W”SizeGauge(PCF)(PSI)(KSI)DirectionDirectionDirectionDirection1/8.00073.12707521013045221/8.0014.552015034022070311/8.00156.187024050532098411/8.0028.1140035072545513554 5/32.00072.6 5/32.0013.8 5/32.00155.3 5/32.0026.9 5/32.00258.4 3/16.00072.0 3/16.0013.1270752101304522 3/16.00154.45001453302156830 3/16.0025.77702204603009038 3/16.00256.9108028559037511446 3/16.0038.11400350725455135541/4.00071.68520855021111/4.0012.3165451408532161/4.00153.43209023515050241/4.0024.348014032021066291/4.00255.267019041026582351/4.0036.085023549531596401/4.0047.91360340700440130523/8.00071.0301045301273/8.0011.63/8.00152.3165451408532163/8.0023.02607020012543213/8.0034.24601353102006529Other Plascore honeycomb products include:                PAMG-XR1 5056 aluminum honeycomb is a lightweight core material which offers superior strength and corrosion resistance over PAMG-XR1 5052 and PCGA Commercial grade aluminum core. PAMG-XR1 5056 core is made from 5056 aluminum alloy foil and meets all the requirements of MIL-C-7438.        PCGA-XR1 honeycomb is a lightweight core material offering excellent strength and corrosion resistance for industrial applications at low cost. PCGA-XR1 core is made from 3003 aluminum alloy foil.        PN ARAMID Honeycomb is a lightweight, high strength, non-metallic honeycomb manufactured with ARAMID fiber paper (DuPont NOMEX™ or equivalent). The ARAMID paper is treated with a heat resistant phenolic resin. This core material exhibits excellent resiliency, small cell size, low density and outstanding flame properties.        Plascore polycarbonate honeycomb core exhibits a unique cell structure: The core has 3 orientations vs. the 2 orientations common with other cores, making its properties more uniform. Each cell has a tubular form and inherently stable.        Plascore polypropylene honeycomb core exhibits a unique cell structure: The core has 3 orientations vs. the 2 orientations common with other cores, making its properties more uniform. Each cell has a tubular form and inherently stable.        Plascore polypropylene honeycomb is supplied with or without a non-woven polyester veil for better bonding. It is also supplied with or without a film barrier under the polyester veil to limit the amount of resin consumption.        
Euro-Composites offer for sale honeycombs with cell sizes ranging from 3.2 to 19.2 mm and a density of between 24 and 200 kg/m3, in hexagonal and rectangular-celled cores.
Aerospace manufacturers started the use of honeycomb products in airplanes and spacecraft because, pound-for-pound, it's the strongest, most rigid product known. Some of its special uses were the heat shield on John Glenn's space capsule, interior structures for America's first Skylab, and shock-resisting hulls of hydroplanes.
There are many ways to fasten one structure to another. Fasteners are objects that attach one item to another, and create methods by which those attachments are effected. In industrial applications, fastening may be accomplished through needle stitching, anchoring, connecting, locking, welding, riveting, nailing, screwing, adhesive bonding, chemical reaction bonding, magnetic bonding, and the like.
The concept of fastening started with the earliest concepts of interweaving of dissimilar materials, clamps, nailing, screwing and the like. Eventually, man learned to bond with metals, and this led to riveting, bolting and welding. Each of these techniques led to advances in the art of fastening.
An adhesive is a substance used to bond two or more surfaces together. Most adhesives have the advantage of forming a bond by filling in the minute pits and fissures normally present even in very smooth surfaces. Adhesive bonds are economical, distribute the stress at the bonding point, resist moisture and corrosion, and eliminate the need for rivets and bolts. The effectiveness of an adhesive depends on several factors, including resistance to slippage and shrinkage, malleability, cohesive strength, and surface tension, which determines how far the adhesive penetrates the tiny depressions in the bonding surfaces. Adhesives vary with the purpose for which they are intended. Such purposes now include the increasing use of adhesives in aerospace applications. Synthetic adhesives used both alone or as modifiers of natural adhesives, perform better and have a greater range of application than the natural products. Most of them form polymers, huge molecules incorporating large numbers of simple molecules to form strong chains and nets that link surfaces in a firm bond. Thermosetting adhesives, which are transformed into tough, heat-resistant solids by the addition of a catalyst or the application of heat, are used in such structural functions as bonding metallic parts of aircraft and space vehicles. Thermoplastic resins, which can be softened by heating, are used for bonding wood, glass, rubber, metal, and paper products. Elastomeric adhesives, such as synthetic or natural rubber cements, are used for bonding flexible materials to rigid materials.
Many aerospace structures are adhesively bonded through the use of thin adhesive films, typically made from a filled thermosetting resin such as an epoxy resin. These films are easier to apply and cleaner to use, and therefore find wide acceptance in applications where neat utilization of the adhesive is a plus factor.
SYNCORE® syntactic foams offered for sale by Henkel Loctite Corporation, d/b/a Loctite Aerospace, Bay Point, Calif., take the place of more expensive prepreg plies in stiffening critical structures. These isotropic foams are composite materials containing preformed microballoons in a thermosetting matrix resin. A wide variety of preformed microballoons and matrices can be combined to make SYNCORE® syntactic foams. Glass is the most common microballoon material of construction, but quartz, phenolic, carbon, thermoplastic and metal-coated preformed microballoons have been used. Epoxies curing at 350° F. (177° C.) and 250° F. (121° C.) are the most common thermosetting matrix resins, but matrices of bismaleimide (“BMI”), phenolic, polyester, PMR-15 polyimide and acetylene or acrylic or vinyl-terminated resins have been used to produce SYNCORE® syntactic foams. As a result of the variety of materials that successfully make SYNCORE® syntactic foams, they are tailorable to a variety of applications. There is a version of SYNCORE® syntactic foams available that will co-cure with all known available heat-cured composite-laminating resins. SYNCORE® syntactic foams allows sandwich core concepts to be used in a thinner dimension than previously possible. The thickness limit on honeycomb cores is approximately 0.125 inch. SYNCORE® syntactic foams are available in 0.007 to 0.125 inch (0.18 mm to 3.2 mm) thickness but can be made in thinner or thicker sheet forms. Other core materials such as wood and sheet foam can be made thin, but are not drapable and generally require an expensive/heavy adhesive film to bond to the partner composite components. In addition, SYNCORE® syntactic foams possess excellent uniformity in thickness which provides the ability to assure quality for the composite in which it is used as a component. SYNCORE® syntactic foams are typically used to replace prepreg plies where the intent is to increase stiffness by increasing thickness.
Designing with SYNCORE® syntactic foams is straightforward because all of the analysis methods that apply to other core materials such as honeycomb apply to it. Flexural stiffness of flat plates and beams increases as a cubic function of thickness allowing a lighter, stiffer lamination than could be made from prepreg plies alone. Since SYNCORE® syntactic foams, on a per volume basis, typically costs less than half of a comparable carbon prepreg, it also leads to a lower cost lamination. This is illustrated by the following:                Adding one ply of 0.020 inch SYNCORE® syntactic foams and eliminating one ply of prepreg does not change the weight or cost significantly, but nearly doubles the flexural rigidity.        Adding one ply of 0.020 inch SYNCORE® syntactic foams and eliminating three plies of prepreg sharply decreases the cost and weight with a small decrease in rigidity.        Adding one ply of 0.040 inch SYNCORE® syntactic foams and eliminating three plies of prepreg provides lower weight, cost and sharply increases rigidity.        The introduction of unidirectional tape allows a further increase in performance at lower cost and weight at nearly the same thickness.        A hybrid tape/fabric/SYNCORE® syntactic foams construction gives a very attractive set of weight and cost savings coupled with a 3.4 times increase in flexural rigidity.        
SYNCORE® syntactic foams have been recommended for thin composite structures in any application where flexural stiffness, buckling, or minimum gauge construction is used. SYNCORE® syntactic foams have been shown to save weight and material cost in carbon fiber composites, and in the case of glass fiber composites to save weight at approximately the same cost. Illustrative applications are disclosed in U.S. Pat. Nos. 4,861,649, 4,968,545, and 4,994,316.
Assembly methods with SYNCORE® syntactic foams is similar to those used with prepregs. Because it is not cured, SYNCORE® syntactic foams are tacky and drapable when warmed to room temperature and is easier to lay-up than a comparable prepreg ply. SYNCORE® syntactic foams can be supplied in supported forms with a lightweight scrim to prevent handling damage when it is frozen. Like prepregs, SYNCORE® syntactic foams require cold storage, usually at 0° F. (−17.7° C.) or below. The various SYNCORE® syntactic foams typically have a room temperature out-time that is much longer than their companion prepregs. SYNCORE® syntactic foams are less sensitive to cure cycle variations than prepreg making the controlling factor the composite cure cycle selection. SYNCORE® syntactic foams will cure void free under full vacuum or low (e.g., about 10 psi) autoclave pressure. SYNCORE® syntactic foams have been cured at up to about 150 psi, without exhibiting balloon crushing.
In a typical application, a sandwich of SYNCORE® syntactic foam and prepreg, such as a thicker layer of SYNCORE® syntactic foam between two thinner layers of prepreg, are held together under heat and pressure to cure the structure into a strong panel. Typical sandwich constructions of this nature are shown in U.S. Pat. Nos. 4,013,810, 4,433,068 and 3,996,654. Such composite structures typically are produced in flat sheets and in separable molds to obtain various desired shapes.
Though SYNCORE® syntactic foams will cure void free under significantly reduced pressure or when put under pressure, it would be desirable to avoid those costly conditions to achieve void reduction. It would be desirable to have a material that has the properties of SYNCORE® syntactic foams but achieves void free construction without costly full vacuum operations or low autoclave pressure systems. These methods are typically batch type operations that materially add to the cost of making the composite.
There are certain applications in which it is desirable to have the properties of a uniform thin drapable syntactic foam film in processing the formation of a laminated composite, yet have the capacity to autogenously expand so as to fill any void space existing in the composite's structure so as to minimize the effects of macro and micro void defects at interlaminate interfaces.
These interlaminar interfacial micro or macro void spaces are magnified by the irregularity of the reinforcing layer of the composite structure. For example, if the composite is of a layer of prepreg-derived carbon fiber reinforced thermosetting resin material, bonded to a syntactic foam, such as a SYNCORE® syntactic foam, the layer containing the prepreg-derived material will have an irregularly shaped surface and the SYNCORE® syntactic foam layer will have a relatively smooth uniform surface. Though the SYNCORE® syntactic foam is tacky and drapable, it is incapable of filling in all of the irregularities of the prepreg-derived layer. Application of a full vacuum or the use of a low-pressure autoclave can be used to significantly reduce the void space, but complete avoidance of micro voids is not readily achievable. Also, conforming SYNCORE® syntactic foams to the irregular surface causes transfer of the irregularity to the opposite surface of the SYNCORE® syntactic foam. Such surface irregularity transfer may be avoided by sandwiching the SYNCORE® syntactic foam using heat and pressure. This re-positions the matrix resin and the microspheres so that the film within the sandwiched structure loses its original uniformity.
It would be desirable to be able to adequately bond a syntactic foam thin film to an irregular surface1 and fill the defects in the surface without transferring the shape of the defects to the unbonded side of the film. It would also be desirable to be able to adequately bond a syntactic foam thin film to a surface and, without the use of vacuum or low-pressure autoclaves, fill the micro voids with the syntactic foam without repositioning the film's matrix resin and microspheres. Such advantages are achieved by the use of SYNSPAND® expandable films, described below. 1 Such a surface is one that may contain undulations, cracks, large pores, warpage, and the like defects.
There is a body of technology directed to fabricating expandable thermoplastic resinous material. For example, U.S. Pat. No. 2,958,905, is directed to a method of making foam structures from particulate expandable granular thermoplastic resinous material containing in the particles a blowing agent for further expansion of the particles. A considerable number of thermoplastic resins are described as suitable for this purpose. The blowing agents are the conventional ones recommended for that application. The expandable granular thermoplastic resinous material may be admixed with a thermosetting resin to generate on curing the exotherm needed to expand the expandable granular thermoplastic resinous material. The resulting mass can be poured into a mold to make a number of products. The '905 patent indicates that the expandable granular thermoplastic resinous material can be formed in the presence of non-expandable filler materials such as staple fibers from a variety of sources, and the mixture fed to a mold for forming an expanded product. The resulting foamed product may be designed to adhesively bond to a fabric layer for reinforcement of the foamed product. The density of the foamed product can be controlled by the amount of the expandable material fed to the mold. At column 12, lines 5 et seq., the '905 patent speaks to the formation of molded products by charging the mold “with the expandable material in any desired manner including manual filling or pneumatic conveyance thereof.” According to the description at column 12 relating to FIGS. 3 and 4 (see column 12, lines 16-32):                . . . a considerable occurrence of void and hollow spaces occurs between the charged expandable beads 21 in the mass to be fabricated, each of which (in the case of pre-expanded material) is a foam structure containing a plurality of internal cells or open spaces. When the liquid exothermus [sic] substance is added between such interparticle voids, the heat from its spontaneous self reaction causes the beads to expand whereby, as illustrated in FIG. 4, the expanded and fabricated particles 22 force out a substantial portion (and frequently most) of the exothermus [sic] substance excepting for a minor quantity of reacted material 23 which remains, frequently as an interlaced and interlinking network between the expanded particles to assist in holding the expanded, cellular foam particles together. (Emphasis added.)        
U.S. Pat. No. 2,959,508, describes another variation of using expandable thermoplastic particles. Here, the unexpanded particles and the exothermus substance, such as an epoxy resin, are first mixed and then poured into the mold to form a composite foam of the two when the exothermus substance heats up the mixture and causes the blowing agent to volatilize.
Thermosetting resins have in the past had blowing agents incorporated therein (see e.g. U.S. Pat. No. 3,322,700) to form expanded molded products and recently, such types of resin systems have included preformed microspheres in the formation of partial syntactic foam films. These expanded thermosets comprise a more open cellular structure unlike that of syntactic foams, and the inclusion of preformed microspheres does not alter that condition.
There are commercial molding processes that utilize tacky sheets of thermosetting resins and reinforcing material. One such process involves the compression molding of sheet molding compounds (“SMC”). In that process, a thermosetting polyester resin filled with staple glass fiber and low profile thermoplastics, are sheeted out and thickened into a pourable paste retained between release surfaces such as polyethylene film. Chunks of the thickened paste are casually deposited around the surface of the mold by hand, and on closing the mold with heating, the paste is liquefied and it, and its fiber loading, are redistributed around the mold to fill it up and form the desired molded article. In other words, the chunks of sheets of SMC represent a convenient way in which to add a liquefiable moldable material to the mold. This process is presently commercially practiced in a number of industries. Advantages of the process are the convenience of storing moldable mixture and the ease of loading a mold with the molding composition.
An advantage of SYNCORE® syntactic foams for many applications resides in its uniformity of distribution of the microsphere throughout the matrix resin. Such microspheres remain essentially intact throughout the cure cycle. As a result, it is not possible to have the microspheres concentrate at one or more surfaces, or one or more other locations in the final composite. It would be desirable to have a drapable thin film, having the handling qualities of SYNCORE® syntactic foams, but which would allow the production of a syntactic foam having a controllable density gradient that accommodates specific end use applications.
There are a number of applications in which thin film syntactic foam could serve as a seal to preclude the passage of gases and liquids. In some applications, the seal could be subjected to abrasion forces. It would be desirable to have a thin film syntactic foam that can be applied in a manner that allows it to be a sealant to gas or liquid flow in a confined space and be able to withstand abrasive forces. Such advantages are derived from the use of SYNSPAND® expandable films, described below.
U.S. Pat. Nos. 5,234,757, 5,397,611, 5,540,963, and 5,783,272 describe a thin film technology that can be used in forming syntactic foam. Such a thin film is commercially available as SYNSPAND® expandable films from Henkel Loctite Corporation, d/b/a Loctite Aerospace, Bay Point, Calif. SYNSPAND® expandable films combine expansion with syntactic technology to produce an efficient core filler and reinforcement material. SYNSPAND® expandable films comprise a thin, tacky film of incompatible in situ-expandable thermoplastic particles in a thermosettable matrix resin that contains an essentially uniform density and thickness across the breadth of the film. In its optimum form SYNSPAND® expandable films provide closed cell expansion, see U.S. Pat. No. 5,783,272. SYNSPAND® expandable films can be laid up in a honeycomb core and each film that is laid up is homogeneously expanded therein to reinforce and stiffen the honeycomb structure at that portion of the structure, without unduly adding weight to the structure. SYNSPAND® expandable films are sold in 1 foot by 2-feet sheets and rolls stock (18 inches by 25 lineal feet) in thicknesses of 50 and 100 mils. SYNSPAND® expandable films may contain preformed microspheres as well, such as those that are used in making SYNCORE® syntactic foams.
SYNSPAND® expandable films are an excellent product and has gained wide commercial acceptance. However, as the utilizations of such forms of syntactic foams are expanded, there are uses where the thin film qualities of SYNSPAND® expandable films do not conveniently meet the needs of the user. For example, in honeycomb applications, a sheet or sheets of SYNSPAND® expandable films are applied over a plurality of honeycomb open spaces in the core, and with heat and pressure, generally under vacuum, but preferably by use of positive pressure from a heated platen, the SYNSPAND® expandable film is forced into open spaces to which it is contiguous. This requires the SYNSPAND® expandable films to flow into the honeycomb structure and assume some dimensional structural similarity to that of the open space of the honeycomb into which it flows. This technique insures that the SYNSPAND® expandable films is in contact with the honeycomb surface over which it is originally laid, either the top or bottom surface of the honeycomb structure, and the walls of the open cells of the honeycomb structure. It is inevitable that some amount of the SYNSPAND® expandable films is left behind on that surface. Further heating cause the matrix resin to flow and sag into the core of the honeycomb structure, and the in situ expandable particles to expand, preferably before gelation of the matrix resin, primarily in the “z” direction (see the discussion in the bridging paragraph at columns 9 and 10 of U.S. Pat. No. 5,397,611), and allows for curing the expanded SYNSPAND® expandable films in the honeycomb. This serves to reinforce and stiffen the honeycomb structure without unduly adding weight to the structure. However, the small amounts of SYNSPAND® expandable films left on the top or bottom surface of the honeycomb are going to be expanded as well. In addition, sagging of the resin causes it to contact the walls of the honeycomb core before expansion, thus expansion not only is in the “z” direction, but also starts from the wall to fill the center of the core; which is opposite to the type of expansion that one would desire. The small amount of SYNSPAND® expandable films left on the top or bottom surface can be ameliorated to some degree by applying an adhesive film over that surface into which the residual SYNSPAND® expandable films can be solubilized.
A deficiency in SYNSPAND® expandable film technology resides in the fact that the top of the film is the primary exposed surface area for expansion. Because the edges of the film possess relatively little of the films surface area, and the molding method of choice forces the edges into contact with the walls of the open cells of the honeycomb structure followed by sagging, expansion must of necessity occur essentially from and in the “z” direction and also from the core walls. This concentration of expansion to essentially a single surface fails to optimize the strength/density/rigidity relationship in such honeycomb structures and does not readily allow reinforcement within the open cells of the honeycomb structure by continuous filament fibers aligned normal to the surface of the honeycomb structure. Under the sagging scenario, expansion in the “z” direction causes the resin to intersect at some part of the open space being filled. Unless the resin contact is seamless by virtue of molten mixing of the different resin film, there is an opportunity for the resin to form an interface within the open space that possesses a lower structural integrity than the remainder of the expanded polymer structure. In addition, SYNSPAND® expandable film technology does not readily allow structural combinations with continuous filament structures in a way that maximizes rigidity and strength to syntactic foam structures incorporated into a traditional honeycomb structure.